Geographical Information

GIS
By:fazil

Geographical Information System - GIS What is GIS? GIS is
an emerging method of data storage and interpretation. GIS is,
simply put a database. It is many tables of data organized by
one common denominator, location. The data in a GIS system
is organized spatially, or by its physical location on the base
map. The information that is stored in the database is the
location and attributes that exist in that base map, such as
streets, highways, water lines, sewers, manholes, properties, and
buildings, etc. each of these items don?t just exist in the
database, the attributes associated with the item is also stored.
A good example of this would be a specific sewer line, from
and arbitrary point A to a point B. Ideally, the sewer line
would be represented graphically, with a line connecting the
two points or something of the like. When one retrieves the
information for that line in particular, the attribute data
would be shown. This data would include the size of pipe, the
pipe material, the upper invert elevation, the downstream
invert elevation, the date installed, and any problem history
associated with that line. This is the very gist of what a GIS
system is. How is a GIS system created? Building a GIS system
from the ground up is a very time consuming and extremely
expensive venture. This is why only large metropolitan areas
have developed or are developing GIS systems. STEP1 -
Determining and acquiring a base map Since a GIS system is
basically a digital map, the extents of the map have to be
determined. Once the area of interest is decided, the base map
has to be built. This is done using aerial photography (digital
orthography). This type of photography is very high definition,
and of consistent scale (all photographs are taken from almost
exactly the same altitude). The photography that is done is
difficult. Times when photographs are able to be taken are few.
For instance, time is limited to the winter months due to less
foliage, and from 10:00am to 2:00pm to reduce the shadows.
STEP 2 - Digital Overlay Once the base maps have been
acquired, the time consuming work begins. Each digital map
section has to be gone over by a person on a computer. That
person outlines each object to be included in the gis system.
These objects are the same as the ones stated previously (like
sewer lines, water lines, etc.), although they are not limited to
those. Along with all of the physical information that is
entered into the database, there is a lot of other physical
information to be added that is not reflected on the aerials,
such as property lines, tax and voting districts, as well as zoning
districts. STEP 3 - Data entry Following the design and
creation of the basic data skeleton, the actual data need be
entered. This includes all of the attribute data for all hydrants,
sewer lines, water lines, properties, buildings, streets, highways,
creeks, etc. this portion of the process is the most time
consuming. The bulk of the data to be entered is on paper, and
there is no easy way to convert it to digital other than manual
data entry. One issue in this step that is worth noting is the
importance of the quality control. As the data is entered,
errors become inherent. In order to keep these at a minimum,
an effective quality control system needs to be in place to
maintain data integrity. STEP 4 - Application Development
Once all the information is entered (which never happens
since the aerial photographs become outdated in about one
month), the core data is placed on a file server for the different
agencies to access. At this stage each agency usually directs
some resources into application development. Since the
structure and organization is very generic, so all agencies can
use it, each entity develops applications for the data that is
better suited for their purposes. For example, the metropolitan
sewer district would focus on the ability to query and store data
relating to the sewer system, since it is their main focus. The
county auditor would begin to restructure its file system and
information management system